3,308 research outputs found
Efficient Parallel Solution of the 3D Stationary Boltzmann Transport Equation for Diffusive Problems
International audienceThis paper presents an efficient parallel method for the deterministic solution of the 3D stationary Boltzmann transport equation applied to diffusive problems such as nuclear core criticality computations. Based on standard MultiGroup-Sn-DD discretization schemes, our approach combines a highly efficient nested parallelization strategy [1] with the PDSA parallel acceleration technique [2] applied for the first time to 3D transport problems. These two key ingredients enable us to solve extremely large neutronic problems involving up to 10 12 degrees of freedom in less than an hour using 64 super-computer nodes
Résolution parallèle efficace de l'équation de transport 3D de Boltzmann pour des problèmes diffusifs
This paper presents an efficient parallel method for the deterministic solution of the 3D stationary Boltzmann transport equation applied to diffusive problems such as nuclear core criticality computations. Based on standard MultiGroup-Sn-DD discretization schemes, our approach combines a highly efficient nested parallelization strategy with the PDSA parallel acceleration technique applied for the first time to 3D transport problems. These two key ingredients enable us to solve extremely large neutronic problems involving up to 10^(12) degrees of freedom in less than an hour using 64 super-computer nodes.Ce papier présente une méthode efficace pour le calcul déterministe d'une solution au problème des équations de Boltzmann pour le transport stationnaire 3D appliqué à des problèmes diffusifs de calcul de criticité dans les coeurs de réacteurs nucléaires. Notre approche, basée sur un schéma de discrétisation standard en multi-groupes Sn-DD, combine une stratégie de parallélisation efficace avec la technique d'accélération parallèle PDSA appliquées pour la première fois à des problèmes de transports 3D. Ces deux ingrédients clés nous ont permis de résoudre des problèmes de neutronique extrêmement large impliquant jusqu'à 10^(12) degrés de libertés en moins d'1 heure sur 64 noeuds d'un super-calculateur
Influence of asperities on fluid and thermal flow in a fracture: a coupled Lattice Boltzmann study
The characteristics of the hydro-thermal flow which occurs when a cold fluid
is injected into a hot fractured bedrock depend on the morphology of the
fracture. We consider a sharp triangular asperity, invariant in one direction,
perturbing an otherwise flat fracture. We investigate its influence on the
macroscopic hydraulic transmissivity and heat transfer efficiency, at fixed low
Reynolds number. In this study, numerical simulations are done with a coupled
lattice Boltzmann method that solves both the complete Navier-Stokes and
advection-diffusion equations in three dimensions. The results are compared
with those obtained under lubrication approximations which rely on many
hypotheses and neglect the three-dimensional (3D) effects. The lubrication
results are obtained by analytically solving the Stokes equation and a
two-dimensional (integrated over the thickness) advection-diffusion equation.
We use a lattice Boltzmann method with a double distribution (for mass and
energy transport) on hypercubic and cubic lattices. Beyond some critical slope
for the boundaries, the velocity profile is observed to be far from a quadratic
profile in the vicinity of the sharp asperity: the fluid within the triangular
asperity is quasi-static. We find that taking account of both the 3D effects
and the cooling of the rock, are important for the thermal exchange. Neglecting
these effects with lubrication approximations results in overestimating the
heat exchange efficiency. The evolution of the temperature over time, towards
steady state, also shows complex behavior: some sites alternately reheat and
cool down several times, making it difficult to forecast the extracted heat.Comment: In Journal of Geophysical Research B (2013) online firs
Modeling transport of charged species in pore networks: solution of the Nernst-Planck equations coupled with fluid flow and charge conservation equations
A pore network modeling (PNM) framework for the simulation of transport of
charged species, such as ions, in porous media is presented. It includes the
Nernst-Planck (NP) equations for each charged species in the electrolytic
solution in addition to a charge conservation equation which relates the
species concentration to each other. Moreover, momentum and mass conservation
equations are adopted and there solution allows for the calculation of the
advective contribution to the transport in the NP equations.
The proposed framework is developed by first deriving the numerical model
equations (NMEs) corresponding to the partial differential equations (PDEs)
based on several different time and space discretization schemes, which are
compared to assess solutions accuracy. The derivation also considers various
charge conservation scenarios, which also have pros and cons in terms of speed
and accuracy. Ion transport problems in arbitrary pore networks were considered
and solved using both PNM and finite element method (FEM) solvers. Comparisons
showed an average deviation, in terms of ions concentration, between PNM and
FEM below with the PNM simulations being over times faster
than the FEM ones for a medium including about pores. The improved
accuracy is achieved by utilizing more accurate discretization schemes for both
the advective and migrative terms, adopted from the CFD literature. The NMEs
were implemented within the open-source package OpenPNM based on the iterative
Gummel algorithm with relaxation.
This work presents a comprehensive approach to modeling charged species
transport suitable for a wide range of applications from electrochemical
devices to nanoparticle movement in the subsurface
Disorder and interference: localization phenomena
The specific problem we address in these lectures is the problem of transport
and localization in disordered systems, when interference is present, as
characteristic for waves, with a focus on realizations with ultracold atoms.Comment: Notes of a lecture delivered at the Les Houches School of Physics on
"Ultracold gases and quantum information" 2009 in Singapore. v3: corrected
mistakes, improved script for numerics, Chapter 9 in "Les Houches 2009 -
Session XCI: Ultracold Gases and Quantum Information" edited by C. Miniatura
et al. (Oxford University Press, 2011
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